The invention relates in general to the field of turbocompressors, such as are used in gas turbines (in particular as aircraft engines), for energy generation or in the chemical industry. In particular the invention is directed to a method and apparatus for timely identifying an incipient compressor surge during operation of a turbocompressor so that suitable countermeasures can be taken. The invention also relates to blade damage to a rotor of a turbomachine, such as a steam turbine or gas turbine. The gas turbine may be an aircraft engine or a stationary gas turbine, which have rotors in both the compressor and the turbine.
Turbocompressors generally have a stability limit which is dependent on their power characteristic. If this stability limit is inadvertently exceeded during operation of the turbocompressor (for example as a result of inlet disturbance, temperature changes or dirt), then severe non-stationary disturbances occur (rotating separation, surging), which can rapidly lead to destruction of the machine. When designing the turbocompressor, it is therefore normal to provide a sufficient margin between the operating line and the stability limit, with all disturbances which could reduce the surge limit margin being taken into account as the safety margin. However, a fixed safety margin such as this results in a considerable loss of operating range for the compressor with good efficiency.
In order to further improve the efficiency and/or the power density in modern designs, investigations have been carried out to determine how turbocompressors can be operated safely close to the stability limit. When an incipient surge occurs (when a predetermined minimum surge limit margin is infringed), it is known that the operating line of the compressor can fall rapidly, or the surge limit can be shifted. This may occur, for example, as a result of a blow-off valve being opened and/or as a result of adjustment of guide vanes, and/or as a result of a reduction in the fuel supply. Various approaches have been adopted in order to determine that the surge limit is being approached.
German Patent Document DE 693 25 375 T2 discloses a method for monitoring and controlling a compressor, in which pressure fluctuations within a compressor stage are measured, and their frequency components are analyzed. If at least one characteristic spike occurs in a frequency range which is dependent on the rotation speed and the number of blades, a warning signal is produced as a function of the shape of the at least one spike which has occurred. The warning signal may be used for closed-loop and open-loop control purposes, in order to avoid the incipient critical state by, for example, reducing the load or reducing the fuel injection rate.
U.S. Pat. No. 6,231,306 B1 discloses a control system for preventing flow separation in a turbocompressor. A mean value for the square of the amplitude of a relevant frequency range is calculated from a measurement signal that is determined by a pressure sensor. The mean value is normalized and compared with a threshold value. If the threshold value has been exceeded, either a blow-off valve is opened or the guide vane position is changed.
German Patent Document DE 694 11 950 T2 discloses a method for identification of a surging, in which the engine exhaust gas temperature and the engine compressor rotation speed are evaluated.
There is a need to further improve the known methods in terms of their reliability and/or the complexity required for sensor systems and signal processing.
One object of the invention is accordingly to provide a method for reliably identifying an incipient surging in a turbocompressor in good time, such that suitable measures can still be taken in order to avoid surging.
A further object of the invention aim is to identify blade damage to a rotor of a turbomachine as early as possible.
Another object of the preferred embodiments of the invention is to achieve this aim with as few additional sensors as possible, that is to say with as few sensors as possible which are not already provided in any case for the turbocompressor. Finally, still a further object of the preferred embodiments of the invention is to avoid complex computation operations, in order to achieve a rapid reaction rate (data processing in real time) with relatively little computation power.
According to the invention, these objects are achieved by a method for determination of a warning, by a method for operation of a gas turbine or of a turbomachine, by a turbocompressor and by a gas turbine as described herein.
The invention is based on the fundamental idea of identifying revolving disturbances which occur as the compressor stability limit is approached. In experiments in which the compressor was slowly choked as far as the surge limit, revolving disturbances such as these could be observed as a primary factor in the compressor instability. The speed of revolution in the annular space of the compressor is dependent on the compressor and, in some circumstances, also on the rotation speed. The disturbances may be both long-wave (modal) and short-wave (in the form of so-called spikes).
According to the invention, a combined criterion is provided for the warning. This criterion is composed firstly of a secondary criterion that the characteristic, periodic disturbance patterns occur considerably in the measurement signal of a temperature, pressure or flow velocity sensor, and secondly from the secondary criterion that the measurement signal from the first sensor is correlated with the measurement signal from a second sensor, which is arranged offset with respect to the first sensor in the circumferential direction of the turbocompressor or of the turbomachine. Further temperature, pressure or flow velocity sensors may be provided. The warning is produced as a function of the extent to which these two secondary criteria are satisfied.
The invention provides reliable early identification of surging and blade damage based on the identification of the stated characteristic signal structures, which occur when the operating point approaches the surge limit and in the event of blade damage. The instrumental complexity is low because the at least two sensors which are required are either already present for other reasons in normal compressors, or they can at least be added without difficulties. The computation complexity for determination of the two secondary criteria mentioned above is not particularly great either, in particular because no complex frequency analyses are required. The invention allows a rapid-response surge limit warning or a warning of blade damage to be emitted with relatively little computation power.
In the wording used in the present document, the expression “surging” should be regarded in the widest sense and, in addition to actual surging, also covers rotating flow separation (rotating stalling) in the compressor. The expression “surge limit warning” should accordingly be regarded as any warning signal providing information about incipient flow separation or surging in the compressor.
The at least two temperature, pressure or flow velocity sensors provided according to the invention are arranged offset with respect to one another in the circumferential direction of the turbocompressor or of the turbomachine. They may have a circumferential separation of 180° or else less, for example 90°, 60°, 45° or 30°. Even if more than two temperature, pressure or flow velocity sensors are provided, they do not necessarily need to be arranged with a standard circumferential separation. The at least two sensors are preferably located on a common axial plane of the turbocompressor or of the turbomachine. This may, for example, be the plane in front of the first rotor; however, other planes are likewise possible.
The at least two measurement signals determined according to the invention correspond to the output signals from in each case one of the temperature, pressure or flow velocity sensors. The expression “correspond” does not necessarily mean that they are identical; in fact, the output signal from a sensor may, for example, be scaled (multiplied by a constant or a variable factor) or shifted (added to a constant or variable value, for example in order to remove the mean value) or inverted (multiplied by −1 or formation of the reciprocal), in order to obtain the appropriate measurement signal from it. Furthermore, the measurement signals are preferably digital value sequences, which have been obtained by analog/digital conversion (and possibly further processing steps) from the analog sensor output signals.
According to the invention, a first time offset and a second time offset, respectively, are used for determination of the periodicity value and of the correlation value. In different embodiments of the invention, the first and/or the second time offset are constant (possibly as a function of the compressor type) or are dependent on the respective speed of revolution or other parameters (for example the compressor pressure). The invention is also not restricted to calculation of in each case only one periodicity value and correlation value; in fact, embodiments are also envisaged in which two or more of these values are always calculated and evaluated (typically with different time-offset values or for different measurement signals).
The steps in the method according to the invention are preferably carried out by a programmable device, for example a digital signal processor (DSP). However, implementations are also feasible with hard-wired digital logic or analog implementations. The sequence in which the method steps are enumerated in the claims should not be regarded as any restriction; in fact, these method steps may also be carried out in a different sequence, or entirely or partially in parallel or semi-parallel (interleaved with one another).
In preferred embodiments, the warning is emitted when the product of the periodicity value and of the correlation value exceeds a predetermined threshold value. In other embodiments, rather than forming the product, a different function is used which links the two stated values such that large periodic signal changes and/or a high signal correlation lead to the warning being emitted. The threshold value calculation can in further embodiments be carried out independently for the two values, with the warning preferably being emitted only when both threshold values are exceeded.
In order to calculate the periodicity value and/or the correlation value, the required measurement signals are preferably evaluated using a sliding window with a predetermined (fixed or dependent on the measurement values) window width. The window width governs the required computation complexity and can therefore also be varied depending on the available computation power. The sampling frequency for the sensors and for signal evaluation is in the order of magnitude of 1 kHz to 2 kHz in preferred refinements.
Provision is preferably made for the periodicity value to be calculated as the average value (scaled or not scaled) of the square error between in each case two measurement points, which are shifted by the first time offset with respect to one another, of one of the measurement signals. The evaluated measurement signal is in some embodiments previously subjected to removal of the mean value. In alternative embodiments, the magnitude difference or the cube of the magnitude difference is formed instead of the square error. A pure addition formation process may also be carried out, instead of calculation of the mean value, in alternative embodiments (particularly when the window width and/or the first time offset are/is constant). Overall, the periodicity value is intended to indicate the extent to which structures with strong periodic signal changes occur in the measurement signal.
In order to calculate the correlation value, the mean value of the product of in each case two measurement points, which are offset by the second time offset with respect to one another, of two different measurement signals is calculated in preferred embodiments. In this case as well, an addition process may be carried out in alternative embodiments rather than formation of the mean value, and a different function may be used rather than the product calculation. Overall, the correlation value is intended to indicate how accurately the two measurement signals under consideration match when they are shifted through the second time offset with respect to one another.
In some embodiments of the invention, the warning that is determined is just indicated to a pilot or to some other operator. Preferably, however, an operating parameter of the turbocompressor is changed in reaction to the surge limit warning in a method step which takes place automatically, in order to avoid a compressor surge. For example, a blow-off valve may be opened, or the stator blades of the turbocompressor may be adjusted.
If the turbocompressor is a component of a gas turbine, then the flow can also be stabilized when proximity to the surge limit is identified by thrust nozzle adjustment, blowing in or out, variable guide vane adjustment or fuel modulation, before the compressor becomes aerodynamically unstable.
The stated measures mean that it is possible to operate the gas turbine (for example the aircraft engine) closer to the surge limit in many operating conditions than would be possible with a static surge limit margin. This leads to improved efficiency and to improved fuel consumption characteristics (lower thrust-specific fuel consumption SFC). Even if this option is not exhausted, the operational reliability of the gas turbine is improved because disturbances which would lead to instability without regulation are identified in advance, and are overcome by increasing the surge limit margin in a controlled manner.
When a gas turbine (in particular an aircraft engine) is being newly developed using the invention, the improvements which can be achieved by the invention may be taken into account in order to design the new development for a higher turbine stage load, if required, and/or to optimize the required surge limit margin as a function of the requirement.
In preferred refinements, the turbomachine, the turbocompressor and the gas turbine are developed with features which correspond to the features described above or to the features mentioned in the dependent method claims.